Since a liquid crystal display (LCD) device has advantages of light weight, thin thickness and low power consumption, such an LCD device has been widely used for information handling equipment, such as personal computers and word processors, and video equipment, such as television receivers and car navigation systems, as a display device. Many LCD devices applied to those applications include an illumination unit that projects light from a rear side of a display panel to achieve a bright display picture.
Here, an illumination unit is classified into an edge light system or a directly-under-disposed system depending on its disposition of a light source. The edge light system, for example, includes its light source disposed at an edge of a light guide plate provided opposite to a display panel. The directly-under-disposed system, however, includes a plurality of straight-line light sources of fluorescent discharge tubes or the like provided behind a display panel and a diffusing plate provided between the display panel and the light source.
It is easily achievable to make the directly-under-disposed system much brighter than the edge light system. The former is advantageous because a light emitting surface of the former is more uniform in brightness than that of the latter. Recently, larger size LCD devices have been widely used for TV receivers and much brighter LCD devices are required for them. It is essential to adopt a directly-under-disposed type illumination unit for such demands.
Conventionally the structure disclosed in Japanese Patent Publication No. 2003-5157, for instance, is known as a directly-under-disposed type illumination unit. In the LCD device disclosed in Japanese Patent Publication No. 2003-5157, the illumination unit to project light from the rear side of an LCD panel includes a housing, a light source (lamp), a lamp holder to support the light source, a reflective sheet, an optical diffusing plate to efficiently transfer light projected from the light source to the LCD panel, and a rear cover containing the light source, the lamp holder, the reflective sheet and the diffusing plate.
In the structure of such a conventional illumination unit, when an LCD panel size becomes larger, thermal expansion of the reflective sheet due to heat generated from the light source (lamp) results in a critical problem as to display quality. The expansion of the reflective sheet in the illumination unit interferes with the rear cover, so that the reflective sheet is distorted. Portions distorted in the reflective sheet result in uneven reflection of light, i.e., illumination light projected to the LCD panel becomes uneven in distribution. The deterioration of display quality is critical for any LCD panel and must be improved.